Absorption spectroscopy, in particular with the aid of infrared [radiation], is often used for identifying certain substances, for example in the course of process monitoring. For this purpose, according to the prior art flow cells are used, through which the substances to be detected are led with the aid of carrier liquids. The use of an FTIR ATR flow cell (FTIR: Fourier transformation infrared; ATR: attenuated total reflection) for such process monitoring has been proposed in WO 02/082061 A and WO 2005/124300 A; a sensor element in the form of an ATR crystal, such as a diamond, is used in which total reflection of introduced infrared radiation occurs during the spectroscopy, wherein the total reflection at the boundary surfaces of the ATR crystal is damped (attenuated) as a function of the deposition (sedimentation) of the substances (particles) to be detected. One problem is that the sensor used in the flow cell may become contaminated by the substances, in particular when biotechnological processes or fermentation processes are involved, which must be monitored; the substances produced in a fermentation medium and their contents, as well as microorganisms and their physiological status, may be monitored. For cleaning the ATR crystal, a chemical cleaning method has typically been used in the past in which the flow cell is rinsed with appropriate cleaning substances. However, removing such polymeric or organic deposits (often also referred to as biofilm) from the optical sensor, i.e., in particular from the diamond window of an ATR element, using wash solutions requires relatively long cleaning times. This problem of film formation on the sensor surface has also been addressed in U.S. Pat. No. 5,604,132 A, in which physical cleaning in the form of stripping is indicated; however, this method is likewise complicated, and problematic with respect to the sensitive crystal surfaces.
According to WO 2005/124300 A cited above, it is further provided to generate ultrasonic standing waves for particle manipulation in order to improve the measurement or for cleaning the sensor surfaces. Such ultrasonic fields in the form of “quasi-standing waves” are easily controlled, and when piezo converters (piezo transducers) are used, which is the preferred case, the ultrasonic field is easily controllable with regard to frequency and amplitude by appropriately adjusting the electrical signal which actuates the particular piezo converter. To obtain the desired spatial standing wave, the emitted ultrasonic wave may be reflected on the opposite side of the flow cell, at the ATR sensor, for example, the returning wave being superimposed by the emitted wave, thus forming the standing wave. In this standing wave the envelope curve of the amplitude is stationary in the direction of sound propagation, i.e., is constant over time.
For such an ultrasonic standing wave, axial (primary) acoustic radiation forces act on deposited particles which are present in the flow cell, in particular at the sensor surface; the effect on the particles, for example cells such as yeast cells, is such that these particles are pushed in the direction of the pressure nodes of the standing wave field. Accordingly, the particles suspended in the carrier liquid are concentrated in planes parallel to the piezo transducer surface, namely, in the pressure node plane(s). Since the ultrasonic field is generally stronger, for example, in the middle than at the edge due to the fact that the ultrasound generation in the piezo transducer is not totally homogeneous, transversal primary acoustic radiation forces also act on the particles, which, depending on the concentration of the particles in the pressure node planes, results in forces being exerted on the particles in the direction of specified locations (for example, the axis of the flow cell) inside these planes, resulting in intensified agglomeration of the particles at these locations in the pressure node planes; as a result, a type of chain of particle agglomerates is obtained. This concentration of the particles, i.e., these agglomerates, remain(s) as long as the ultrasonic field is switched on. When the ultrasonic field is deactivated, the particles are transported from the flow cell due to the liquid flow.
In carrying out the spectroscopy according to this known technique, the particle agglomerates also have a shorter sedimentation time than individual cells, so that quicker measurements, in shorter successive intervals, are possible as the result of the shortened sedimentation time; on the other hand, the interfering sedimentation of particles on the sensor surface (biofilm formation) is counteracted. Tests have shown that when the particles are held in a floating manner above the sensor element for a period of 30 s, for example, by activating the ultrasonic field before falling onto the sensor surface, and agglomerated, the resulting particle agglomerate sediments significantly faster than individual particles.
Although this known technique according to WO 2005/124300 A has proven to be suitable, it is disadvantageous that a dedicated flow cell is used, which makes it necessary for a bioreactor, for example, which is used for carrying out a process to be monitored, to deliver the substances to be measured in dedicated lines of the flow cell, from which the substances are once again returned to the bioreactor. This means an additional outlay of equipment, and a further disadvantage results in the described sedimentation technique, according to which the sedimentation of particles on the surface of the ATR sensor must be awaited before a specific measurement is carried out. In addition, a change in the chemical parameters could result due to the transport of the substances to the measuring location outside the bioreactor.
On the other hand, a device has been proposed in DE 43 33 560 A1 for continuous spectroscopic analysis according to the principle of attenuated total reflection, in which an ultrasound source situated in the vicinity of the measuring surface is used to clean product residues from the measuring surface of an ATR element. In particular, the ultrasound source generates ultrasonic waves which are directed toward the measuring surface. In one embodiment having a built-in probe which is fixed to a reactor wall via a flange connection, a reflector for increasing the ultrasonic intensity is associated with the ultrasound source; however, a standing wave is not generated.
Lastly, U.S. Pat. No. 5,604,132 A describes monitoring of a chemical process with the aid of an infrared detector provided in a dedicated circuit, whereby samples of the reaction mixture are periodically injected into a liquid carrier stream and supplied to the IR detector. However, this has nothing to do with FTIR absorption spectroscopy.